Polystyrene foamed products have excellent performances as a heat-insulating material and a packaging and cushioning material and find wide applications in various fields. However, they have a low recovery of compression strain and withstand heat at a temperature of 70.degree. to 80.degree. C. at the highest. These defects may be eliminated by using polypropylene foams or crosslinked polyethylene foams. It is difficult, however, to produce starting foamed particles for forming these polyolefin foamed products since blowing agents used for polyolefin resins have a high speed of dissipation. Any foamed particles that may be obtained have a low rate of expansion with a bulk density of from 0.1 to 0.5 g/cm.sup.3 at the highest.
In an attempt to solve this problem, there has been proposed a process for producing high-expansion polypropylene foamed particles having a bulk density of from 0.05 to 0.07 g/cm.sup.3, which comprises dispersing polypropylene resin particles in a dispersing medium, e.g., water, in a closed vessle, maintaining the dispersion at a pressure above the saturated vapor pressure of the dispersion and at a temperature above the softening point of the polypropylene to thereby penetrate the dispersing medium into the polypropylene resin particles, and then jetting the dispersion from the inside of the closed vessel under high pressure into the atmosphere, as disclosed in U.S. Pat. No. 3,770,663 corresponding to Japanese Patent Publication No. 2183/74.
According to the above-described process, water used as a dispersing medium also serves as a blowing agent, and high-expansion products having a bulk density as high as 0.016 to 0.04 g/cm.sup.3 cannot be obtained.
Attempts have been made to overcome this problem by using a combination of water and a volatile organic blowing agent as a blowing agent. That is, a process for producing polyolefin resin foamed particles which comprises dispersing polyolefin resin particles in water in a closed vessel, feeding a blowing agent into the closed vessel, heating the dispersion to a temperature above the softening point of the polyolefin resin particles but below a temperature 20.degree. C. higher than the melting point of the polyolefin resin particles while maintaining the pressure within the closed vessel at the vapor pressure of the blowing agent or a higher pressure, opening a discharge port provided in the closed vessel below the liquid level, and releasing the dispersion containing the polyolefin resin particles impregnated with the blowing agent into an atmosphere having a lower pressure than the pressure within the closed vessel has been proposed as described in Japanese Patent Application (OPI) Nos. 12035/82, 25336/82-90027/82, 195131/82, 1732/83, 23834/83, 25334/83, 33435/83, 55231/83, 76229/83, 76231/83, 76232/83, 76233/83, 76234/83 and 87027/83 (the term "OPI" herein used means "unexamined published application").
This process can provide polypropylene foamed particles having a bulk density of from 0.026 to 0.06 g/cm.sup.3. Further, it is described therein that this process can also be applied to polyethylene particles and crosslinked polyethylene particles instead of propylene copolymer particles.
Into the polypropylene foamed particles thus obtained is introduced air or nitrogen under pressure to impart secondary foamability to them, and the foamed particles are filled in a steam mold, heated to mutually melt-adhere, and cooled to form a final foamed product, such as a bumper, a container, etc.
Further, the polypropylene foamed particles are compressed by a pressurized nitrogen gas, and the compressed particles are filled in a mold, heated to mutually melt-adhere, and cooled to obtain a final foamed product.
In the molding to obtain a foamed product, a cooling time is a great factor for productivity of a product. According to currently employed process, the productivity is ensured by cooling the foamed product after melt-adhesion to a temperature about 20.degree. to 30.degree. lower than the crystallization temperature of the resin at which the foamed product does not undergo deformation and then opening the mold to take out the product.
Therefore, a smaller difference between a melting point and a crystallization temperature of the resin makes a cooling time shorter and is, therefore, very advantageous from the standpoint of improving productivity.